Added closeport guard

Author: Vzor-

src/main/java/jssc/SerialPort.java

@@ -1182,6 +1182,7 @@ public synchronized boolean removeEventListener() throws SerialPortException {
      * @throws SerialPortException if exception occurred
      */
     public synchronized boolean closePort() throws SerialPortException {
+        if (!portOpened) return false;
         boolean returnValue;
         //removeEventListener calls setEventsMask, and must occur before calling closePort
         try {